A unit that forms an optical system base unit for concentrator photovoltaic includes, for example, a primary lens being a convex lens, a secondary lens being a sphere lens, and a power generating element (for example, see PATENT LITERATURE 1 (FIG. 8)). As the power generating element, a solar cell having high power generation efficiency is used. Sunlight is concentrated by the primary lens, to be incident on the secondary lens, and then is further concentrated by the secondary lens, to reach the power generating element. Such a configuration allows much light energy to be concentrated onto a small power generating element, whereby power can be generated with high efficiency. A large number of such concentrator photovoltaic units are arranged in a matrix shape to form a concentrator photovoltaic module, and then, a large number of the modules are arranged in a matrix shape to form a concentrator photovoltaic panel. The concentrator photovoltaic panel forms a concentrator photovoltaic apparatus, together with a driving device for causing the panel to perform tracking operation while facing the sun.
The secondary lens is provided in order to concentrate light energy onto a small area, thereby reducing the area necessary for the expensive power generating element as much as possible, but rather, in order to reduce deviation in tracking the sun and reduce influence of an error in the mounting position of the secondary lens relative to the primary lens, thereby increasing concentrating accuracy. That is, in the case of the primary lens alone, when the optical axis is displaced due to tracking deviation or an error in the mounting position, a part of the concentrated light goes outside the light receiving surface of the power generating element. In this case, power generation efficiency is reduced. Thus, in order to guide light to the power generating element even if some displacement of the optical axis has occurred, the secondary lens being a sphere lens is provided (for example, see PATENT LITERATURE 1 (FIG. 10c)).
There has also been proposed a technology in which, as a secondary lens, a lens having a special shape including two arc-like protrusions is used to increase power generation efficiency (for example, see PATENT LITERATURE 2 (FIG. 6, paragraph [0006])). Also in this case, light that would go outside the light receiving surface of the power generating element with a primary lens alone can be guided to the light receiving surface by the secondary lens.